Electron discharge apparatus



April 10, 1945. w. A. DEPP ELECTRON DISCHARGE APPARATUS Filed Sept. 29,1942 2 Sheets-Sheet 1 TD COAHPOL CIRCUIT I m CONTROL CIRCUIT C mvs/vronA A W A. Of PP NCONTROL CIRCUIT A A T TORNEV April 10, 1945.

FIG. 6

FIG. 7

E POTENTIAL- VOLTS ANOOE POTENTIAL VOLTS ANODE POTENTIAL VOL TS w. A.DEPP 2,373,175

ELECTRON DISCHARGE APPARATUS Filed Sept. 29, 1942 52 Sheets-Sheet 2 I Il l J 20 40 00 00 I00 PHOIE ELECTRODE POTENTIAL FOR TRAN! E I? VOL TSPROBE ELECTRO0E POTENTIAL EOR TNANSFER- VOL TS l l 20 (0 6'0 00 I00PROBE ELECTRODE POTENTIAL FOR TRANSFER J'flTS lNVENTOR n. A. DE PP mam5.1M

A 7' TORIVEV .1 ment of several, for: example'three, potential con-UNITED STATES PATENT OFFICE ELECTRON DISCHARGE APPARATUS Wallace A.Depp, Elmhurst, N. Y., assignor to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application September 29, 1942,Serial No. 460,085

6 Claims. (01. zen- 27.5)

This invention relates to electron discharge apparatus and moreparticularly to electron discharge devices of the shield or screen gridtype.

Shield or screen grid type electric discharge devices comprise, ingeneral, a cathode, an anode,

a control electrode and a shield electrode mounted in an envelope havinga filling 01- a gas or other ionizable medium. The control electrode iseffective to initiate a discharge between it and the cathode and theshield electrode performs the "dual function of shielding the anode fromthe control gap region and of efiecting transfer of the discharge to themain gap, that is the gap between the cathode and the anode.Satisfactory shielding of the anode in such devices entails the use of ashield electrodehaving a relatively large area in the main dischargepath. Consequently, the current drawn by the shield electrode isrelatively large, for example of the order of 20 per cent of the maingap current. As a result, when a. plurality of such devices is operatedwith the shield: electrodes in parallel and supplied from a highimpedance source, as, for example, in switching circuits in automatictelephone systems, when one of the'devices operates false operation ofother of the devices may occur because ofthe increase in the potentialof the shield electrodes thereof above the transfer value.

Further, in such devices of conventional design, the transfercharacteristics are dependent largely upon the parameters of the device,particularly the kind and pressure of the gas filling oi the envelopeand the spacing of the shield electrode and the cathode so that theoperating electrode electric discharge devices of the shield or screengrid type whereby a high impedance transfer control is realized:

Increase the flexibility of operation of shield grid. type electricdischarge devices whereby a range of transfer characteristics for anyparticular device is obtainable;

Reduce variationsin the transfer characteristic of such devices, with.changes in the control gap current; and

Obtain multipotential control of themain gap dischargelin such deviceswhereby the establishditions is prerequisite to the initiation of adischarge across the main gap.

In one illustrative embodiment of this invention, an electric dischargedevice comprises a cathode, a control electrode adjacent the oathode, ananode, and a shield electrode between the cathode and the anode. Theshield electrode may be a disc extending transversely with respect tothe main discharge gap and having a central restricted aperture inalignment with the cathode and anode. An auxiliary or probe electrode isprovided for controlling the transfer of the discharge to the main gap.In one form, the aurailiary or probe electrode is a slender wire or redhaving one end immediately adjacent the sparture in the shield electrodeand extending men ally outwardly adjacent the shield electrode.

The invention and'the features thereof will be understood more clearlyand fully from the following detailed description with reference to theaccompanying drawings in which:

Fig. 1 is a perspective view of an electric discharge deviceillustrative of one embodiment of this invention, a portion of theenclosing vessel being broken away to show the electrodes more clearly;

Fig. 2' is a side view, partly in section, of the device shown in Fig.1;

Fig. 3 is a circuit diagram illustrating one man'- ner of operating theelectric discharge device shown in Figs. 1 and 2; V

Fig. 4 is a circuit diagram illustrating parallel operation of aplurality of devices constructed in.

sap current; and

Fig. 7: is a graphillustrating the transfer characteristics fordifferent shield. electrode currents in an. electric discharge deviceconstructed inaccordance with this invention.

Referring now to the drawings, the electric discharge device:illustrated in Figs, 1 and 2 compris'es an enclosing vessel [0 having afilling of an ionizable medium, for example a mixture of per cent neon,5 per cent argon ata pressure of about; 40 millimeters of mercury, andprovided with a stem- I l from which a cathode l2, a control electrodel3 and a shield electrode M are supported and in which leading-inconductors 50 are sealed.

The shield electrode I4 is annular, for example a metal disc providedwith a restricted central aperture I5, and is supported by rigidmetallic uprights or wires l6 embedded in the stem I I, the electrodebeing secured to the uprights by metallic angle pieces Il and theuprights being encased in insulating, for example glass, sleeves 18. Thecathode l2 may be a disc, as shown, secured by an angle piece I9 to arigid metallic support 20 surrounded by an insulating, e. g. glass,sleeve 2| and embedded in the stem II, the surface of the.

cathode l2 facing the shield electrode l4 being coated with a material,such as a mixture of barium and strontium oxides, having good electronemission properties. The control electrode I3 is a metallic wire Or rodoverlyin and inclined with respect to the emissive surface of thecathode l2 and is supported by a rigid metallic conductor 22 encased inan insulating, e. g. glass, sleeve 23 and embedded in the stem l l.-

Sealed to and extending through the end wall of the vessel l and axiallyaligned with the aperture I is a wire rod anode 24. .Extending fromimmediatelyadjacent the aperture l5 and inclined with respect to theshield electrode 14 is a wire rod auxiliary or probe electrode 25, whichis sealed to and extends through the enclosing vessel I0.

During operation of the device, one manner of operation beingillustrated in Fig. 3, the anode 24 ismaintained at a positive potentialwith respect 'to the cathode 12 as by a battery 26, the shield electrodel4 being connected to a point in the anode circuit through aresistance-21 which may be, for example, of the order .of 1 or more meg-;ohms. The control electrode I3 is biased at a small potential, positivewith respect to the cathode l2, as by a battery 28 and the auxiliaryorprobe electrode is biased positive with respect to the cathode [2 asby a battery 29 in series with a high resistance 30, for example of theorder of "100,000 ohms.

The anode potential is insufiicient to initiate a discharge between thecathode I2 and anode 24 for a given bias upon the control electrode 1 3but is large enough to sustain such a discharge after the initiation ofa discharge between the control electrode l3 and cathode l2 as a resultof the.

application to the control electrode I3 of a suitvable potential by wayof the control circuit and the application of a potential to the probeelectrode 25 sufiicient to effect transfer of the dis- The potential-ofthe screen electrode l4 is less than that requisite to'sustain adischarge "across the cathode-shield electrode gap. The auxiliary orprobe electrode is efiective to control charge.

the transfer of the discharge from the cathodecontrol electrode gap tothe main gap between the cathode and the anode. In general, for a givencontrol gap current and shield electrode potential,

there is a fairly critical probe electrode potential below whichtransfer will be prevented and above which transfer is permitted,

,Thetransfer characteristic of any device is dependent upon, inter alia,the control gap current and the shield electrode potential, as shown inFigs. 5 and 6. In Fig. ,5 are shown the transfer and the same shieldelectrode potentials for a control gap current of 200 microamperes. Itwill be noted that for both values of control gap current, the transfercharacteristic can be varied over a fairly wide range by changing thepotential of the shield electrode, so that substantial flexibility inthe operation of the device is achieved and any particular device issuitable for a variety of applications. 10

Stated in another way, a single device constructed in accordance withthis invention enables realization, by virtue of the variation of thetransfer characteristic with shield electrode potential, of thecharacteristics of a number of devices.

Because of the form of the probe electrode, the current drawn thereto issmall so that a high impedance control, by the probe electrode, isachieved. As an example, it may be noted that in a particular device ofthe construction shown in Figs. 1 and 2, the current to the probeelectrode was approximately 0.1 milliampere as compared with a shieldcurrent of approximately 2 milliamperes in a shield grid device ofcomparable power rating and of conventional construction. Thus, in thedevice constructed in accordance with this invention the impedance ofthe control element was about twenty times that of a device ofconventional design.

, When the gap between the cathode l2 and control electrode I3 isconducting, some current from .thedischarge is drawn to the shieldelectrode l4 and at high shield electrode potentials this cursmall. Thisis illustrated in Fig. 7, in which curve X shows the transfercharacteristic of a device of the construction shown in Figs. 1 and 2with a control gap current of approximately 40 microamperes, a shieldcurrent of approximately 40 microamperes and a shield electrodepotential, of approximately 70 volts and curve Y is the transfercharacteristic of the same device with thesame shield electrodepotential but with a control gap current of about 200 microamperes and ashield electrode current of about 260 microamperes.

Curve Z shows the transfer characteristic for the same device withsubstantially the same shield electrode potential and shield electrodecurrent as in the case of curve Y but with a control gap current ofapproximately 40 microamperes. From curves Y and Z it will be noted thatif the shield electrode current is'maintained at a fixed value, a greatchange in the control gap current is ac companied by only asubstantially negligible variation in the transfer characteristic.Specifically, in the example given, a change in control gap current from200 to 40 microamperes results in a change of but about 5 per cent inthe transfer characteristic.

A substantially constant current to the shield electrode may be realizedby constructing and arranging the shield electrode and cathode so thatthe potential requisite for breakdown of the gap therebetween is high,for example of the order of per cent of the main gap breakdownpotential, and connecting the shield electrode through a highresistance, of the order of l or more megohms toa potential source suchthat the shield electrode potential is slightly less than the breakdownvalue.

The high impedance control feature noted hereinabove is of particularadvantage in circuits, such as crossbar switching circuits in automatictelephone systems, wherein a plurality of devices is operated with theirmain and control gaps in parallel. In conventional screen grid devices,the screen current is large as noted heretofore and, thus, when severalsuch devices are operated in parallel, when one device operates there isdanger of the others operating due to the increase of the potentials ofthe screens thereof above the transfer value. In devices constructed inaccordance with this invention, however, the shield current is small asnoted heretofore, so that false operation of devices connected inparallel when one device is energized to become conductive, isprevented.

A typical circuit for parallel operation is shown in Fig. 4. The maingaps, that is the gaps between each cathode I2 and the associated anode24, are connected in parallel to the load circuit and the controlelectrodes [3 also are connected in parallel, through series resistances3|, for example of the order of 100,000 hms, to the control circuit A,which may include, for example, a periodic interrupter, not shown. Theshield electrodes M are biased at a positive potential below thetransfer value by the battery 26 to which they are connected through thehigh resistances 21, which may be, for example, of the order of 50,000ohms. The auxiliary or probe electrodes are connected to individualcontrol circuits B and C through the resistances 30, which may be of theorder of /2 megohm. The particular potential upon each probe electrodeto cause transfer of the discharge to the main gap after the control gapbecomes conducting will be dependent upon the shield electrodepotential, as is apparent from what has been said heretofore.

It will be noted that the device requires that the potentials of threeelectrodes be above certain values before a discharge across the maingap may occur so that three controls are provided. That is, initiationof a discharge between the cathode l2 and the anode 24 requires firstthat the control gap, i. e., the gap between the oathode l2 and controlelectrode l3 break down, secondly that the shield electrode be at aparticular potential, less than the sustaining potential of the shieldelectrode-cathode gap to prevent transfer to the shield electrode, andthirdly a particular probe electrode potential to efiect transfer to themain gap after the control electrode-cathode gap becomes conducting. Thedevice, therefore, may be used'to advantage in interlocking circuits.

Although a specific embodiment of this invention has been shown anddescribed, it will be understood that it is but illustrative and thatvarious modifications may be made therein without departing from thescope and spirit of this invention as defined in the appended claims,

What is claimed is:

1. Electron discharge apparatus comprising a gaseous discharge deviceincluding a cathode and an anode spaced to define a main discharge gap,a shield electrode between said cathode and said anode, means forproducing a discharge adjacent said cathode, and means for transferringthe discharge to said main gap including a slender auxiliary electrodehaving a portion immediately adjacent said shield electrode.

2. An electric discharge device comprising an enclosing vessel having anionizable medium therein, a shield electrode within said vessel andhaving an aperture therein, a cathode on one side of said shieldelectrode, an anode on the other side of said shield electrode, meansfor producing a. discharge in a region adjacent said cathode, andatransfer control electrode between said anode and said shield electrodeand extending into immediate proximity to said aperture.

3. An electric discharge device comprising an enclosing vessel having anionizable medium therein,'a' cathode and an anode within said vessel andspaced to define a main discharge gap, a shield electrode between saidcathode and said anode and having a restricted aperture therein, acontrol electrode adjacent said cathode, and a probe electrode having aportion in immediate proximity to said aperture.

4. An electric discharge device comprising an enclosing vessel, havingan ionizable medium therein, a cathode and an anode within said vesselspaced to form a discharge gap, a trigger control electrode adjacentsaid cathode, means shielding said anode from said control electrodecomprising an annular metallic member imperforate except for arestricted aperture therein, and a transfer control electrode betweensaid anode and said shield electrode, said transfer control electrodehaving one end immediately adjacent said aperture.

5 An electricdischarge device-comprising an enclosing vessel having anionizable medium therein, an anode and a cathode within said vesselspaced to form a main discharge gap, a shield electrode extending acrosssaid gap, said shield electrode being imperforate except for an aperturetherein, means for establishing a discharge in a region adjacent saidcathode, and means for effecting transferof the discharge to said gapincluding a slender probe electrode having one end immediately adjacentsaid aperture.

6. An electric discharge device comprising an enclosing vessel having anionizable medium therein, a shield electrode within said vessel andhaving a restricted aperture therein, a disc cathode to one side of saidshield electrode, an anode on the other side of said shield electrodeand aligned with said cathode through said aperture, a metallic rodcontrol electrode adjacent said cathode, and a rod probe electrodebetween said shield electrode and having one end adjacent said aperture.

,- WALLACE A. DEPP.

